Re: [PATCH] net: Update netdev_alloc_frag to work more efficiently with TCP and GRO

[Eric Dumazet <eric.dumazet@gmail.com>]

On Tue, 2012-06-19 at 17:43 -0700, Alexander Duyck wrote:
> This patch is meant to help improve system performance when
> netdev_alloc_frag is used in scenarios in which buffers are short lived.
> This is accomplished by allowing the page offset to be reset in the event
> that the page count is 1.  I also reordered the direction in which we give
> out sections of the page so that we start at the end of the page and end at
> the start.  The main motivation being that I preferred to have offset
> represent the amount of page remaining to be used.
> 
> My primary test case was using ixgbe in combination with TCP.  With this
> patch applied I saw CPU utilization drop from 3.4% to 3.0% for a single
> thread of netperf receiving a TCP stream via ixgbe.
> 
> I also tested several scenarios in which the page reuse would not be
> possible such as UDP flows and routing.  In both of these scenarios I saw
> no noticeable performance degradation compared to the kernel without this
> patch.
> 
> Cc: Eric Dumazet <edumazet@google.com>
> Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
> ---
> 
>  net/core/skbuff.c |   15 +++++++++++----
>  1 files changed, 11 insertions(+), 4 deletions(-)
> 
> diff --git a/net/core/skbuff.c b/net/core/skbuff.c
> index 5b21522..eb3853c 100644
> --- a/net/core/skbuff.c
> +++ b/net/core/skbuff.c
> @@ -317,15 +317,22 @@ void *netdev_alloc_frag(unsigned int fragsz)
>  	if (unlikely(!nc->page)) {
>  refill:
>  		nc->page = alloc_page(GFP_ATOMIC | __GFP_COLD);
> -		nc->offset = 0;
>  	}
>  	if (likely(nc->page)) {
> -		if (nc->offset + fragsz > PAGE_SIZE) {
> +		unsigned int offset = PAGE_SIZE;
> +
> +		if (page_count(nc->page) != 1)
> +			offset = nc->offset;
> +
> +		if (offset < fragsz) {
>  			put_page(nc->page);
>  			goto refill;
>  		}
> -		data = page_address(nc->page) + nc->offset;
> -		nc->offset += fragsz;
> +
> +		offset -= fragsz;
> +		nc->offset = offset;
> +
> +		data = page_address(nc->page) + offset;
>  		get_page(nc->page);
>  	}
>  	local_irq_restore(flags);
> 

I tested this idea one month ago and got not convincing results, because
the branch was taken half of the time.

The cases where page can be reused is probably specific to ixgbe because
it uses a different allocator for the frags themselves.
netdev_alloc_frag() is only used to allocate the skb head.

For typical nics, we allocate frags to populate the RX ring _way_ before
packet is received by the NIC.

Then, I played with using order-2 pages instead of order-0 ones if
PAGE_SIZE < 8192.

No clear win either, but you might try this too.